Wednesday, February 27, 2013

This is the March 2013 outlook. This outlook will not include a graphical outlook. Rather, a region-by-region summary will be included in this article.

Dates are shown at the bottom of this image as (day/time), so 4/12 would be March 4th at 12z (6 AM CT)

We're going to start with something called the Arctic Oscillation, or AO. The Arctic Oscillation involves the strength of a massive low pressure system in the Arctic that we call the polar vortex. Think of how you used to swirl your straw around in your drink in restaurants when you were a kid. If you swirled it fast, you could form a tornado-like vortex. If you only swirled it a little, the attraction to the center of the cup and the vortex is weak. This can be applied to the polar vortex. When the polar vortex (swirling your straw around) is weak, cold Arctic air is released down to lower latitudes, and can penetrate into the US. This is known as the negative phase of the AO. In the positive phase, a stronger than normal polar vortex (swirling your straw around fast) is observed. This locks up the cold air in the Arctic and provides for a warmer pattern in the United States. If we look above, we see the long range forecast of the Arctic Oscillation from the American ensemble forecasting system. The black line with intermittent circles is what is called the control run of this ensemble system (an ensemble member that does not have its initial parameters changed, unlike other ensemble members), and the green lines illustrate the mean forecast. The blue lines are the maximum and minimum of all the ensemble members' forecasts for the Arctic Oscillation. Analyzing the forecast above, we see that the AO is negative at this time and has been negative recently. This promotes stronger chances of cold air due to the weak polar vortex losing its grip on cold Arctic air. Looking ahead to the future, we see there is a strong consensus for this negative AO to continue into the middle of March. However, at the end of this forecast, the ensemble mean's green lines spike towards neutral territory, and this would indicate a strengthening polar vortex and thus less chances for cold air. As far as I can see, the first half of March has fair potential to be chilly in parts of the US. I will address this further at the end.

Now shown above is the forecast for the North Atlantic Oscillation, or NAO. The same rules about colors and types of lines in this forecast apply. The NAO has a positive and negative phase (in the following description, the positive and negative phase word colors above will correlate with word colors in the next few sentences). When there is high (low) pressure over Greenland, the NAO is said to be negative (positive). As a result, low (high) pressure forms over the Eastern US, and the subtropical jet stream, which helps Nor'easters form, is strengthened (weakened). Thus, when the NAO is negative, one can anticipate better chances for cold and storms in the East US, while the positive NAO brings about warmer weather. Looking at the forecast above, once again from the American ensembles, we see that we are currently in a weak negative phase that will be intensifying in coming days. Looking towards the longer-term, we see a tendency to go back to that medium-weak negative phase, and this could continue through the first half of March. Like the Arctic Oscillation, we see a spike towards more neutral and positive phase territory at the end of this forecast, which could mean warmer times to come. I want to point out that future forecasts of the NAO could be more negative than what we're seeing above. Based on my extrapolation of forecasts over the North Hemisphere, I feel that the negative NAO could be enhanced more than what is being depicted, but only time will tell if that comes to fruition.

For our last forecasting indice, we have the Pacific North American index, or PNA. The PNA has a positive and negative phase. In the positive phase, high pressure forms in the West US. In accordance with Newton's Third Law of Motion, an action will have an equal and opposite reaction, meaning high pressure in the West US will provoke low pressure in the Central and East US. The positive PNA is the favored stage for storms and cold in these two regions. In a similar fashion, when the PNA is negative, low pressure dominates the West US. Going back to the Third Law of Motion, high pressure then tends to form in the East US. This type of pattern gives the North Plains the big winter storms, a feature we have seen much of the winter (the PNA was negative during much of the winter, so the Plains storm train can be attributed to that). The forecast of the PNA starts out with us in a solid modest positive phase. This looks to continue into the first days of March before we go into neutral territory to round out the first week of March. Beyond this, we drop well into negative territory with the PNA, and you know what that means: storms diverted away from the East US, warmth dominates. And look at those blue maximum-minimum forecast lines- for a couple days at the end of the second week in March, the forecast is off the charts, while the maximum forecast barely breaks neutral territory. That is not a good sign for those in the East and Central US wanting cold weather for March, but an excellent sign for the warm-weather lovers in those areas.

This is a long range forecast from the most recent American ensembles, and this time they are forecasting 500 millibar height anomalies. Reds and oranges tell of high pressure, while blues indicate low pressure. I outlined the three indices we talked about above- the AO, NAO and PNA. We see very strong, blocking high pressure from the Bering Sea into the Arctic, something the American ensembles have been hinting at for quite a while now. There is no low pressure in that area, meaning the polar vortex has been exiled from its domain. On a related note, large low pressure in northeast Asia shows where the polar vortex may have been shifted to. The fact that the polar vortex isn't even allowed in its domain means cold air is much more likely to flow south and into lower latitudes, the extent of which is to be determined. We see a solid negative NAO over Greenland, shown by high pressure in that region. Now, I said how I believed the NAO may be more negative than what is currently being forecast, and the reason for this is because of that strong -AO. There is certainly plausible evidence that the negative Arctic Oscillation's high pressure could influence and strengthen the high pressure over Greenland. I will be watching for this in the next week or two to see how it plays out. As far as the negative PNA goes, it would be strengthened by high pressure in the Bering Sea into the Gulf of Alaska. As a result, you can see a small shade of orange in the South. As small as it is being shown to be, I can assure you the high pressure would be much stronger if this forecast comes true.

After reconciling these atmospheric factors and adding in a few more complicated indices, this is my March 2013 forecast by region.

Pacific Northwest: Starts more or less warmer than normal, gradually turns stormier and colder.Southwest: Continued instances of stormy weather, possibly increasing after mid-month.South Plains: Turbulent temperatures in first half of March, gradually turning warmer.Southeast: Chilly start gives way to warm end.Northeast and Mid-Atlantic: Bouts of cold and chances for snow give way to warmer end to March.Ohio Valley: Chances for cold and snow lessen as month progresses; turning warmer.Midwest: Chilly start to month ends up moderating as month goes on. Storm track shifts north.North Plains: Cold start, stormy end. Wintry region of the month.

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